As I said it does nothing to make it inoperable but it can certainly cause problems with the slide releasing on it's own or failing to lock back. I'm going by what I saw in the video and what I saw is the slide releasing on it's own with no pressure on it, with mention of on two separate occasions taking sandpaper to the slide lock. The only time I have ever seen a slide release all on it's own is from someone slamming a magazine in too rough.
Just to be clear, I wasn't 'filing' on my gun. It was sandpaper to just smooth it out and round the corner. It barely even did anything to it.
Sandpaper and files serve the same purpose, it doesn't matter what you used both can take off just as much material as the other. Even though it may appear you took barely anything off sometimes that's all it takes. Alot of people attempt home trigger jobs on Smith & Wesson revolvers using a very fine india stone, which is much less abrasive than sandpaper, and plenty of people have taken just a bit too much of the sear on the rear of the trigger which can cause the hammer to fall forward on it's own with just a bit of pressure. It doesn't take much to take off fractions of an inch which is sometimes all it takes.